Silver halide photography was well established commercially before the discovery of X-radiation. It was immediately recognized that silver halide photographic elements could be used for X-ray imaging (i.e., as radiographic elements), with medical diagnostic imaging being soon recognized as a important application. However, there were some problems Image contrast was low and X-radiation, particularly in large doses, was soon recognized to present a medical risk.
Investigation revealed that low image contrast resulted from a significant portion of X-radiation passing through a patient being scattered. The solution to this problem was to develop X-ray collimating grids. These grids, usually an array or two perpendicular arrays of linear slats formed of an X-ray opaque material, such as lead, absorb X-radiation that has been deflected in passing through the patient before the X-radiation reaches the radiographic element to be exposed. Examples of X-ray collimating grids are provided by Freeman U.S. Pat. No. 2,133,385, Stevens U.S. Pat. No. 3,919,559, Albert U.S. Pat. No. 4,288,697, and Moore et al U.S. Pat. No. 4,951,305.
The reduction of patient exposure to X-radiation was addressed as soon as the medical risk was appreciated. Silver halide emulsion layers are much less efficient in capturing X-radiation than in capturing light. Only about 1 to 2 percent of X-radiation received on exposure is absorbed by a silver halide emulsion layer. By coating silver halide emulsion layers on opposite sides of a transparent film support (hereinafter referred to as dual coating) X-radiation absorption can be doubled, thereby halving patient exposure.
A still more dramatic reduction in patient exposure was realized by developing X-ray intensifying screens. The function of these screens is to absorb an imagewise pattern of X-radiation and to emit light to the silver halide emulsion layer or layers of a radiographic element. This typically reduces patient exposure to X-radiation by a factor of about 20.
In medical diagnostic radiology the most efficient imaging systems are those that employ a dual coated radiographic element mounted between a front and back pair of intensifying screens. The front screen is intended to expose a silver halide emulsion layer unit on the front side of the support while the back screen is intended to expose a silver halide emulsion layer unit on the back side of the support.
Although used extensively for many years because of their high imaging efficiency, such systems were recognized to produce less than optimum image sharpness, attributable to crossover. Crossover is the term used to indicate exposure by an intensifying screen of the silver halide emulsion layer unit on the opposite side of the support. When the light emitted by the front screen, for example, is not absorbed by the adjacent front emulsion layer unit and passes through to be absorbed by the back emulsion layer unit, the longer light transmission path permits a larger lateral offset between the point of X-ray absorption and the point of light absorption by the back emulsion layer unit than would have occurred if absorption had occurred in the front emulsion layer unit. This larger lateral offset reduces image sharpness.
There have been over the years many attempted solutions to the crossover problem. These are summarized in Research Disclosure, Item 18431, August 1979, Section V. Cross-Over Exposure Control. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England. The first step toward a practical and acceptable solution to the problem of crossover occurred with the construction of front and back emulsion layer units employing spectrally sensitized high tabularity emulsions, first described by Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426. High tabularity emulsions are those in which tabular grains account for greater than 50 percent of total grain projected area and which satisfy the relationship: EQU ECD/t.sup.2 &gt;25
where
ECD represents the equivalent circular diameter of the tabular grains in micrometers (.mu.m) and
t represents the thickness of the tabular grains in .mu.m.
Dual coated radiographic elements constructed as taught by Abbott et al are capable of reducing crossover to levels below 20 percent.
The second step toward eliminating the problem of crossover was the addition of processing solution bleachable dye layers beneath the front and back spectrally sensitized high tabularity emulsion layer units, taught by Dickerson et al U.S. Pat. Nos. 4,803,150 and 4,900,652. Following the teachings of Dickerson et al the first practically attractive dual coated radiographic graphic elements with low (&lt;10%) crossover levels were achieved. The Dickerson et al dual coated radiographic elements, in fact, can allow crossover to be eliminated entirely, the term "zero crossover" being applied to such dual coated radiographic elements.
Once practical low and zero crossover dual coated radiographic element constructions were available, it became possible to produce independent image records in the front and back emulsion layer units, allowing further flexibility and improvement in imaging capabilities. Asymmetrical assemblies of dual coated low crossover radiographic elements and intensifying screens are illustrated by Bunch et al U.S. Pat. No. 5,021,327, Dickerson et al U.S. Pat. No. 4,994,355, Dickerson et al U.S. Pat. No. 4,997,750, and Dickerson et al U.S. Pat. No. 5,108,881.